(a) Field of the Invention
The present invention relates to tuners relates more particularly to a direct broadcasting satellite broad-band tuner which comprises an auto threshold control demodulator in which the auto threshold control circuit of the auto threshold control demodulator detects high frequency signal from the baseband signal, which is received from the demodulator circuit of the auto threshold control demodulator, and then converts the detected high frequency signal to DC voltage through a DC detector, so as to change the amount of electric current passing through the diode of the variable bandwidth circuit of the threshold control circuit, and therefore the receiving threshold of the tuner is regulated according to the change of weather condition and the specification of the antenna used, the receiving sensitivity of the tuner is improved, and the noise index is lowered.
(b) Description of the Prior Art
During the transmission of direct broadcasting satellite, audio and video signals are properly arranged at the ground transmitting station through a multiplexer, then modulated to an intermediate frequency signal and then amplified and turned into a radio signal for transemitting through a transmitting antenna. The ground receiving station has a low-noise broad-band receiver to receive radio signals. Upon receipt of a radio signal, the signal is amplified and turned into an intermediate frequency signal and then the intermediate frequency signal is further demodulated by a demodulator circuit to a baseband frequency signal containing a video signal and an audio signal. When the ground receiving station receives a useful electromagnetic wave signal from the atmosphere, interference waves and noises from the atmosphere may interfere with the receiving of the signal or pass into the signal, causing the reliability of the communication affected.
Furthermore, regular broadband satellite receiving tuners can receive weak signals well only when the set bandwidth and the used receiving antenna are matched. However, if the level of the satellite signal is changed and the specification of the receiving antenna can not match with such a change, the signal becomes out of the threshold of the tuner, and therefore noise signal will enter the tuner to interfere with its receiving operation, causing the quality of the picture of the TV affected.